The Northern Ireland secretary, John Reid, has warned of a return to violence if the Ulster Unionist party carries out its threat to pull out of the Northern Ireland executive.

Two weeks ago, members of the party's ruling council voted to force their leader, David Trimble, to quit the executive in January unless the IRA fully disarms by then.

Speaking to Guardian Unlimited Politics ahead of his party conference speech today, Mr Reid said he had heard "the death knell" of the agreement being rung out on many occasions, even in the two years he had been in his post.

"But I won't hide from you that it faces grave difficulties because of the deadline imposed by the Ulster Unionists," he warned.

"If it were to fall and if, God forbid, we were to go back conflict, all that would happen is that a lot more people would die."

He called on critics of the deal to "get real" and accept it was the only way forward.

He also rounded on unionist critics of the agreement for claiming that there were alternatives on offer. And Mr Reid made it clear he would use his conference speech to stress that the government's commitment to the Good Friday agreement remains unchanged.

"There are people going round implying that there is some fantasy solution that people can achieve on their own - because no one is going to negotiate with them - but that thought is a myth," he said.

The secretary of state implied that Mr Trimble could have done more to sell the benefits of the deal to his party.

He also said he understood why some Unionists found it difficult to back peace while the IRA remained associated with terrorist violence.

"In David Trimble's case he has stuck in there, following a relatively enlightened path among some of the Unionist leaders, and I understand that it's difficult to sustain that amid allegations that the IRA are still involved in terrorism in Colombia, that they are involved in the burgling of the special branch headquarters."

"But ultimately we are going to proceed with the government's commitments under the Good Friday agreement. We are not going to go back," he said.

"I am utterly convinced of one thing. The basis on which the Good Friday agreement was constructed was in addressing those problems in the history of Northern Ireland, the social and constitutional problems as well as the military problems, that have been unaddressed for centuries."